/**
 * Implements hook_field_widget_settings_form().
 */
function {{ machine_name }}_field_widget_settings_form($field, $instance) {
  $widget = $instance['widget'];
  $settings = $widget['settings'];

  if ($widget['type'] == 'text_textfield') {
    $form['size'] = array(
      '#type' => 'textfield',
      '#title' => t('Size of textfield'),
      '#default_value' => $settings['size'],
      '#element_validate' => array('element_validate_integer_positive'),
      '#required' => TRUE,
    );
  }
  else {
    $form['rows'] = array(
      '#type' => 'textfield',
      '#title' => t('Rows'),
      '#default_value' => $settings['rows'],
      '#element_validate' => array('element_validate_integer_positive'),
      '#required' => TRUE,
    );
  }

  return $form;
}